For the safe operation of a combined-cycle turbine plant, it is necessary for the rotational speeds to be determined and monitored. In general, the speed of a combined-cycle turbine power plant is at a constant frequency of 50 Hz or 60 Hz. This speed can be exceeded under certain circumstances, which can be referred to as an overspeed. When this overspeed exceeds a critical value, a protection mechanism should take place in that measures are taken and further increase in the rotational speed is prevented. This is generally achieved by switching off the supply of steam to the steam turbine and switching off the supply of fuel to the gas turbine. In this case, therefore, the steam turbine will be tripped after the gas turbine.
EP 2 372 482 A2 discloses a method and a system for testing an overspeed protection system of a turbomachine.
DE 299 08 581 U1 discusses a device for monitoring the operational safety of a turbine in the event of load shedding.
FR 2 947 300 A1 discloses a method for testing a turbomachine.
Hitherto, the test mechanisms for overspeed protection of steam turbine and gas turbine plants could be monitored by setting the triggering limit value for the overspeed protection mechanism to a lower speed than the operating speed. In test operation, it is possible to exceed this lower test speed and to check whether the overspeed protection mechanism works.
It would however be desirable to also carry out this test operation with original triggering limit values. That means that the overspeed protection mechanism should be checked proceeding from the operational speeds. This is also desirable given that in certain countries such an overspeed protection, which is to take place with original triggering limit values, is prescribed by law. In the case of single-shaft plants, such a check can be performed only for the gas and steam turbines together, taking into account the permissible configuration parameters.